1. Field of the Invention
This invention relates to dyes used for the formulation of photoresists, especially acid hardened photoresist formulations having a capability of providing highly resolved features of submicron dimension.
2. Description of the Prior Art
Photoresists are photosensitive films used for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque to activating radiation and other areas transparent to activating radiation. The pattern in the photomask of opaque and transparent areas defines a desired image that may be used to transfer the image to a substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York (1988).
Known photoresists can provide features having resolution and size sufficient for many existing commercial applications. However for many other applications, the need exists for new photoresists that can provide highly resolved images of submicron dimension. One class of recently developed photoresists capable of enhanced resolution is known as the "acid hardened resists". These resists typically comprise a resin binder, a photoacid or photobase generator and one or more other materials that will result in curing, cross-linking or hardening of the composition upon exposure to activating radiation and heating as necessary to complete the cure. Acid hardening photoresists have been developed for imaging at essentially all conventional wavelengths such as g-line, i-line and deep ultraviolet (DUV).
A preferred acid hardening photoresist composition comprises a photoacid generator, a resin binder, and an amine-based cross-linker such as a melamine-formaldehyde resin exemplified by the Cymel resins available from American Cyanamid. These acid-hardening resists are described in numerous publications including European Patent Applications Nos. 0 164 248 and 0 232 972 and in U.S. Pat. No. 5,128,232, each incorporated herein by reference for their disclosure of conventional acid hardening photoresist compositions and processes for their use. The photoresists of these citations were developed primarily for DUV imaging.
Highly useful acid hardening photoresist compositions are disclosed in U.S. Pat. No. 5,128,232 to Thackeray et al. The patent discloses, inter alia, the use of a resist resin binder that comprises a copolymer of non-aromatic cyclic alcohol units and phenol units. The disclosed photoresists are also particularly suitable for exposure to deep ultraviolet (DUV) radiation. As is recognized by those in the art, DUV radiation refers to exposure of the photoresist to radiation having a wavelength in the range of about 350 nm or less, more typically in the range of about 300 nm or less.
Acid hardening resist compositions such as those disclosed in the above referenced documents provide fine line, high resolution images. However, imaging equipment most prevalent in the industry is adapted for i-line or g-line exposure and the acid hardening resist compositions of the above references are largely insensitive to these exposure wavelengths. Consequently, efforts have been made in the industry to formulate acid hardening photoresist compositions that may be imaged by i-line and g-line exposure.
In U.S. Pat. Nos. 3,954,475 and 3,987,307, both incorporated herein by reference, there is disclosed a class of vinyl-halomethyl-s-triazine compounds capable of photolysis and halogen acid free radical generation upon excitation at a radiation within the wavelength of from about 330 to 700 nanometers, a range that includes both i-line and g-line wavelengths. It is disclosed in these patents that the aforesaid halogenated triazines may be used for formulation of light sensitive compositions. Photoresists utilizing the triazine photoacid generators have been formulated for i-line and g-line imaging. Such photoresists are disclosed in the literature, for example in U.S. Pat. No. 4,189,323 and in European Patent Applns. 0 458 325; 0 519 298; and 0 621 509, each incorporated herein by reference. The photoresists disclosed in the patent and the European applications typically comprises an alkali soluble resin binder such as a phenolic resin, a crosslinking agent and the halomethyl triazine.
Regardless of the imaging radiation for the photoresist composition, it is extremely important that the photoresist exhibit uniform, constant line width, feature size, feature shape and thickness when patterned on a reflective rough semiconductor substrate or over reflective uneven topography. In this instance, light is reflected at an angle from the areas of the substrate into the photoresist causing localized regions of the photoresist to receive an additional dose of imaging radiation, which produces an undesirable profile shape and poor edge acuity after development. The loss of profile shape can severely limit the ability to successfully survive subsequent processing steps such as the etching of the underlying substrate. Uneven edge acuity can have highly deleterious effect on the electrical properties of a manufactured semiconductor device.
It is known in the art to add a dye to a photoresist composition to absorb stray reflections and thus diminish undesirable profile shapes giving excellent line width control and unexposed film thickness retention. In the J. Electrochemical Society: Solid State Science and Technology, Vol. 133, pp. 192-196 (1986), T. R. Pampalone and F. A. Kuyan describe the effect of the addition of Macrolex 60 dye to positive photoresist compositions. They found that the required exposure dose was increased by 220 to 520%. The sidewall angle of this composition was rather poor, about 60.degree.-70.degree., indicating only moderate resolution. In Solid State Technology, pp. 125-130 (January, 1988), C. A. Mack described absorption properties of two dyed photoresist compositions in an article titled "Dispelling the Myths About Dyed Photoresist." The dyes described were Coumarin 314 and Macrolex 60. Neither dye is soluble in developer. The analysis reported in the above reference indicates that dyed resist compositions may not significantly improve lithographic processing.
U.S. Pat. No. 4,626,492 claims a photoresist composition containing 10-20 percent of a trihydroxybenzophenone and 0.1 to 3.0 percent dye, together with sensitizer and a specific novolak resin composition. The dyes claimed include anthroquinones, coumarins, diphenylmethanes, triphenylmethanes, phenanzines, oxazines, xantheses and phenylazo(4-dialkylamino)benzene. The function of the trihydroxybenzophenone is to increase the dissolution rate of this composition, thus enhancing the light sensitivity of the photoresist. However, the trihydroxybenzophenone additive has the significant disadvantage of being an in vitro mutagen (TSCA Report 8E HQ-0484-0510).
Japanese Patent No. 5,142,538 claims photoresist compositions containing selected phenyl- and naphthyl-azo compounds, including (4-phenylazo)- resorcinol. However the (4-phenylazo)resorcinol component has insufficient absorbency at 436 nm to be useful as the only dye compound in the composition when a mercury g-line wavelength is used to expose the resist coating.
Regardless of the dye used, problems are encountered by the addition of a dye to a photoresist composition. The dye in low concentration inadequately reduces reflected light from the substrate. In higher concentration, the dye competes with the photoresist components for the light thus decreasing sensitivity of the photoresist composition. Other problems include undesirable reaction with resist components and poor solubility in the photoresist composition.